Problem: In chemistry class, the girl to boy ratio is $5$ to $8$. If there are a total of $39$ students, how many boys are there?
Solution: A ratio of $5$ girls to $8$ boys means that a set of $13$ students will have $5$ girls and $8$ boys. A class of $39$ students has $3$ sets of $13$ students. Because we know that there are $8$ boys in each set of $13$ students, the class must have $3$ groups of $8$ boys each. There is a total of $24$ boys in chemistry class.